Touru Yamauchi

Enhanced and inhibited spontaneous emission in GaAs/AlGaAs vertical microcavity lasers with two kinds of quantum wells

We investigate enhanced and inhibited spontaneous emission effects in a vertical λ‐microcavity structure having two kinds of quantum wells (QWs) with the thicknesses of 76 and 114 A, measuring both photoluminescence intensity and carrier lifetime. The 76 and 114 A QWs are placed at the maximum and at the nodes of the emitted standing wave in the microcavity, respectively. When the λ‐microcavity mode is tuned to the quantized band‐gap energy of the 76 A QWs (enhanced condition), the PL intensity is enhanced compared with the case that the cavity mode is tuned to the quantized band‐gap energy of the 114 A QWs (inhibited condition). In addition, the increase of the carrier lifetime is also observed under the inhibited condition. These results demonstrate existence of enhanced and inhibited spontaneous emission effects in the microcavity structures.

Crystal growth of the two-dimensional spin gap system SrCu2(BO3)2

Abstract Our high-quality large single crystals of SrCu 2 (BO 3 ) 2 have been successfully grown by a travelling solvent floating zone (TSFZ) method using LiBO 2 as a solvent. We present the growth condition and the result of the temperature-dependent magnetic susceptibilities measured along the directions parallel and perpendicular to the tetragonal c -axis.

Pressure-induced superconductivity in the iron-based ladder material BaFe2S3.

All the iron-based superconductors identified so far share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square-lattice structures but also in ladder structures. Yet iron-based superconductors without a square-lattice motif have not been found, despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ∼120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity.

Efficient photocarrier injection in a transition metal oxide heterostructure

An efficient method for doping a transition metal oxide (TMO) with hole carriers is presented: photocarrier injection (PCI) in an oxide heterostructure. It is shown that an insulating vanadium dioxide (VO2) film is rendered metallic under light irradiation by PCI from an n-type titanium dioxide (TiO2) substrate doped with Nb. Consequently, a large photoconductivity, which is exceptional for TMOs, is found in the VO2/TiO2:Nb heterostructure. We propose an electronic band structure where photoinduced holes created in TiO2:Nb can be transferred into the filled V 3d band via the low-lying O 2p band of VO2.

Anomalous magnetizations in single crystalline SrCu2(BO3)2

The magnetic properties of a two-dimensional spin-gap compound SrCu 2 (BO 3 ) 2 , realizing the Shastry-Sutherland model, were investigated using high-quality single crystals. The analysis of the temperature-dependent magnetic susceptibility revealed a spin gap of 34 K. The magnetization curve at 0.4 K possibly has, in addition to the already established one-quarter and one-eighth plateaux, another quantized plateau at one-tenth of the full Cu moment, which is one of the plateaux predicted by Miyahara and Ueda.

COEXISTENCE OF DOUBLE ALTERNATING ANTIFERROMAGNETIC CHAINS IN (VO)2P2O7 : NMR STUDY

Nuclear magnetic resonance (NMR) of 31P and 51V nuclei has been measured in a spin-1/2 alternating-chain compound (VO)_2P_2O_7. By analyzing the temperature variation of the 31P NMR spectra, we have found that (VO)_2P_2O_7 has two independent spin components with different spin-gap energies. The spin gaps are determined from the temperature dependence of the shifts at 31P and 51V sites to be 35 K and 68 K, which are in excellent agreement with those observed in the recent inelastic neutron scattering experiments [A.W. Garrett et al., Phys. Rev. Lett. 79, 745 (1997)]. This suggests that (VO)_2P_2O_7 is composed of two magnetic subsystems showing distinct magnetic excitations, which are associated with the two crystallographically-inequivalent V chains running along the b axis. The difference of the spin-gap energies between the chains is attributed to the small differences in the V-V distances, which may result in the different exchange alternation in each magnetic chain. The exchange interactions in each alternating chain are estimated and are discussed based on the empirical relation between the exchange interaction and the interatomic distance.

Observation of Metal–Insulator Transition in Hollandite Vanadate, K2V8O16

We have synthesized hollandite vanadate, K 2 V 8 O 16 , by a solid-state reaction under 4 GPa at 1473 K for one hour, and investigated its structural and electromagnetic properties. We found a meta...

Ultraviolet light selective photodiode based on an organic–inorganic heterostructure

We show a perfect ultraviolet (UV) light selective and sensitive photovoltaic cell easily fabricated. The device consists of a water-soluble p-type semiconducting polymer PEDOT-PSS: poly(3,4-ethylenedioxythiophene) doped by poly(4-styrenesulfonate) film deposited on a Nb-doped titanium oxide (SrTiO3:Nb) substrate. For the PEDOT-PSS/SrTiO3:Nb heterostructure, the photosensitivity at zero bias for the UV light with L=1 mW/cm2 is estimated to be 0.05 A/W, corresponding to a quantum yield of 16% electron/photon. This device is only activated in the UV region (λ<390 nm), and then, it exhibits a large response to the UV–B light (290–320 nm).

Neutron Diffraction Study on the Antiferromagnetic Insulating Ground State of β-Na0.33V2O5

The magnetic structure of the antiferromagnetically ordered state of a quasi-one-dimensional conductor β-Na 0.33 V 2 O 5 below T N =24 K was examined by neutron diffraction. The presence of magnetic Bragg reflections at ( h , k ±1/6,0), h + k =odd below T N indicates that the amplitudes of the magnetic moments at the V sites in three types of V–O chains are modulated with a period of 3 b along the chain direction (∥ b ). On the other hand, it was confirmed that the lattice modulation along the b direction occurs at ∼244 K and at ∼131 K with a period of 2 b and 6 b , respectively. Although the latter structural transition had been attributed to a “charge ordering” transition, which would result in a charge disproportionation in the V 3 d orbitals with a period of 6 b , the 3 b periodicity in the amplitude of magnetic moments strongly suggests that the charge disproportionation occurs with a period of 3 b , not 6 b . The 6 b lattice modulation developing below 131 K is therefore attributed to an instability...

Hall effect of iodine in high pressure

The Hall coefficient of metallic iodine is measured in pressures up to 74 GPa at room temperature and in 30 GPa at 4.2 K. The carrier of the pressure-induced metallic iodine is found to be a hole in agreement with the theoretical prediction.